Aluminum electrolytic capacitors have several important commercial uses. For example, they are used: (a) for general purpose smoothing of signals (ripple current); (b) for energy storage in power supplies; (c) in pulse applications that release large bursts of energy in a short period of time (e.g., flash attachments for cameras, strobes and implantable defibrillators); and (d) specialty applications such as in starting motors for automobiles.
Aluminum electrolytic capacitors are typically fabricated from two strips of aluminum foil (99.99% purity), about 50 μm in thickness with a sheet of porous electrolytic paper interposed between them and wound in the form of a cylinder. One of the aluminum strips is coated with an aluminum oxide layer that acts as the dielectric on the anode. The surface of the other strip is maintained in the metallic state and functions as the cathode. The completed winding is inserted in an aluminum can where the porous paper is vacuum impregnated with the electrolyte (e.g. adipic acid, ammonium pentaborate or ethylene glycol/ammonia). After the capacitor has been sealed in the can, it is re-formed by subjecting it to a DC potential that is sufficient to repair any damage to the oxide layer created during fabrication. Patent application 20030223178, which is incorporated herein by reference teaches an aluminum electrolytic capacitor suitable for use in defibrillators.
The capacitance of an aluminum electrolytic capacitor can be determined from the following relationship:
  C  =            κ      ⁢                          ⁢              ɛ        o            ⁢      A        d  where C is the capacitance, κ and ∈o are the dielectric constant and permittivity, respectively for alumina, A is the surface area of the anode and d is the thickness of the alumina layer. It is evident from inspection of this equation that most of the parameters are constant and that in order to increase capacitance, one must enhance the surface area of the anode.
The main approach to increasing the surface area of the anode is by electrochemical etching in a chloride solution with alternating or concurrent DC and AC currents. It has been found that by following this protocol it is possible to enhance the surface area of the aluminum foil by almost two orders of magnitude. Unfortunately, the electrochemical etching technique generates an undesirable wide pore size distribution and it is impossible to create the desired surface architecture to give the optimum capacitor performance. Ideally, one would like to produce a surface having pores of about 1 μm in diameter with a nearest neighbor spacing of about 10 μm. It is possible to achieve this goal by using laser or lithography/etching techniques, however, these tend to be very expensive approaches. There is therefore a need in the art for a method for producing aluminum electrolytic capacitors with an anode having increased capacitance and a substantially uniform pore size.